This application relates to and incorporates herein by reference Japanese Patent Application No. 11-41651 filed on Feb. 19, 1999.
1. Field of the Invention
The present invention relates to a stick-type ignition coil device which is directly mountable in a plug hole of an internal combustion engine.
2. Related Art
Stick-type ignition coil devices as proposed in JP-A10-289831 (U.S. patent application Ser. No. 09/023,613 filed Feb. 13, 1998) must be sized under the limitation that it is fitted in a narrow plug hole of an internal combustion engine. A resinous insulating material fills in the ignition coil device to ensure electrical insulation between various members closely disposed in the ignition coil device. Spools for windings are shaped in an elongated cylindrical form and disposed coaxially around a stick-shaped central core. Each spool is preferably as thin as possible not to enlarge the outer diameter of the ignition coil device. Glass fibers are admixed in a resin base material as a reinforcing material to restrict plastic deformation of thinned spools. Further, a rubber material may be admixed in the resin base material to increase toughness of the spool.
However, micro voids tend to occur around the glass fibers due to difference in the linear thermal expansion coefficients between the resinous base material and the glass fibers, when the spool is molded from an admixture of the resinous base material and the glass fibers. Further, the rubber material which has a lower thermal decomposition temperature tends to sublimate due to electrical discharges to cause voids, if the rubber material is admixed in the resin base material. These voids will enable the discharges to occur from the surface of the spool to the voids, thus causing treeing which is a kind of dielectric breakdown. If treeing grows to cause the dielectric breakdown in the spool, the spool will lose its insulating function. If treeing further passes through the resinous insulating material and grows to bridge a high voltage part and a low voltage part in the ignition coil device, a secondary coil of the ignition coil device will be unable to generate a required high voltage.
Further, because the resinous insulating material not only ensures electrical insulation but also cements the various members to one another, the members having different linear thermal expansion coefficients are subjected to restraining forces when expanding and contracting in accordance with changes in surrounding temperature. Thus, the spool tend to distort and tend to crack in the end. Cracks in the spool will cause electrical discharges between adjacent coil wires.
It has therefore been proposed to wind a thin film around the outer periphery of the spool, or to coat the coil wires for enabling a separation between the thin film and the resinous insulating material cementing the coil or for enabling a separation between the coated coil and the resinous insulating material. Thus, the inner peripheral side and the outer peripheral side of the ignition coil can expand and contract independently of each other thereby restricting spool cracking.
However, the electrical discharge concentrates in the voids caused by the separation, thus causing erosion locally on the surface of the spool. The local erosion will enable treeing to grow, resulting in the dielectric breakdown of the spool. Although the continuous part of the thin film is less likely to be eroded by the electrical discharge, the spool is still possibly eroded by the electrical discharge passing through connection parts of the thin film.
It is therefore an object of the present invention to provide an ignition coil device that is capable of restricting treeing caused by electrical discharges in a spool from growing.
According to the present invention, an ignition coil device for internal combustion engines includes a stick-type core, a primary spool disposed coaxially with the core, a primary coil wound around the primary spool, a secondary spool disposed coaxially with the core, a secondary coil wound around the secondary spool, and a resinous insulating material filling a space in those parts. At least one of the spools located between the primary coil and the secondary coil is made of a resin base material admixed with glass fibers and silica. The glass fibers restricts plastic deformation of the spool and silica restrict a growth of treeing in the spool caused by electrical discharges.